A traditional achromatic lens or achromat corrects for primary chromatic aberration by matching the focal points of a red and a blue wavelength. The dispersion is reduced over the visible spectrum, however residual secondary chromatic aberration remains at the green wavelength. Achromatic doublets consist of a crown glass followed by a flint. The crown glass has a positive focal length with low dispersion while the flint has a negative power with high dispersion. The combination of these two glass disparities is designed to match the red and blue wavelengths to a common focus along the optical axis (z-axis) to eliminate primary chromatic aberration.
It is known that diffractive lenses have strong chromatic aberration. See, Donald C. O'Shea, Thomas J. Suleski, Alan D. Kathman, Dennis W. Prather, Difractive Optics: Design, Fabrication, and Test (SPIE Press Book, 2004). We have previously reported on designing and demonstrating adjustable focus diffractive lenses using liquid crystal as the variable index medium. See, P. Valley, D. L. Mathine, M. R. Dodge, J. Schwiegerling, G. Peyman, and N, Peyghambarian, “Tunable-focus flat liquid-crystal diffractive lens,” Opt. Lett. 35, 336-338 (2010); P. Valley, M. R. Dodge, J. Schwiegerling, G. Peyman, and N. Peyghambarian, “Nonmechanical bifocal zoom telescope,” Opt. Lett. 35, 2582-2584 (2010).
Previously, discrete tunable liquid crystal diffractive lenses have been proposed. See, P. Valley, D. L. Mathine, M. R. Dodge, J. Schwiegerling, G. Peyman, and N. Peyghambarian, “Tunable-focus flat liquid-crystal diffractive lens,” Opt. Lett. 35, 336-338 (2010); P. Valley, M. R. Dodge, J. Schwiegerling, G. Peyman, and N. Peyghambarian, “Nonmechanical bifocal zoom telescope,” Opt. Lett. 35, 2582-2584 (2010); S. Sato, A. Sugiyama, and R. Sato, “Variable-Focus Liquid-Crystal Fresnel Lens,” Jpn. J. Appl. Phys. 24 pp. L626-628 (1985); H. Ren, Y. H. Fan, and S. T. Wu, “Tunable Fresnel lens using nanoscale polymer-dispersed liquid crystals,” Appl. Phys. Lett. 83, 1515-1517 (2003); B. E. Bagwell, D. V. Wick, R. Batchko, J. D. Mansell, T. Martiez, S. Serati, G. Sharp, and J. Schwiegerling, “Liquid crystal based active optics,” Proc. SPIE 6289, 628908 (2006)). Continuous tunable liquid crystal refractive lenses are also known. See, H. C. Lin and Y. H. Lin, “An electrically tunable focusing liquid crystal lens with a built-in planar polymeric lens,” Appl. Phys. Lett. 98, 083503 (2011); O. Pishnyak, S. Sato, and O. D. Lavrentovich, “Electrically tunable lens based on a dual-frequency nematic liquid crystal,” Appl. Opt. 45, 4576-4582 (2006); H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, “Tunable-focus flat liquid crystal spherical lens,” Appl. Phys. Lett. 84, 789-4791 (2004). Articles have been published on continuously tunable fluidic lenses. See, D. Y. Zhang, N. Justis, and Y. H. Lo, “Integrated fluidic adaptive zoom lens,” Opt. Lett. 29, 2855-2857 (2004); H. Ren, D. Fox, P. A. Anderson, B. Wu, and S. T. Wu, “Tunable-focus liquid lens controlled using a servo motor,” Opt. Express 14, 8031-8036 (2006); R, Marks, D. L. Mathine, G. Peyman, J. Schwiegerling, and N. Peyghambarian, “Adjustable fluidic lenses for ophthalmic corrections,” Opt. Lett. 34, 515-517 (2009)) with variable focal lengths were demonstrated individually.
At each focal location chromatic aberration is observable when uncorrected. Chromatic aberration is greater in elements with larger dispersion, such as diffractive lenses, and becomes more significant for materials with lower Abbe numbers. Diffractive and holographic lenses have been proposed to replace the traditional achromatic doublets for fixed focal length designs. See, Donald C. O'Shea, Thomas J. Suleski, Alan D. Kathman, Dennis W. Prather, Difractive Optics: Design, Fabrication, and Test (SPIE Press Book, 2004); T. Stone and N. George, “Hybrid diffractive-refractive lenses and achromats,” Appl. Opt. 27, 2960-2971 (1988)). More recently, a useful hybrid variable focal length fluidic/diffractive lens was demonstrated; however, the diffractive component applies a fixed focal length. See, G. Zhou, H. M. Leung, H. Yu, A. S. Kumar, and F. S. Chau, “Liquid tunable diffractive/refractive hybrid lens,” Opt. Lett. 34, 2793-2795 (2009).
It is desirable to provide an improved lens combination with more versatile characteristics.